Current automotive emission regulations necessitate the use of exhaust aftertreatment devices. Oxides of nitrogen (NO.sub.x), for example, can be reduced to nitrogen and water in a selective catalyst reduction device using externally added reducing agents. NO.sub.x conversion in a three-way catalyst operating at stoichiometric air-fuel ratio is commonly greater than 95%. Such high conversion NO.sub.x efficiencies have not been demonstrated in practice in engines in which the exhaust stream is leaner than stoichiometric. Diesel engines and lean gasoline engines operate lean and cannot, therefore, realize the high NO.sub.x conversion efficiencies of three-way catalysts.
Exhaust aftertreatment devices for use in lean operating engine systems tend to have a narrow temperature range in which NO.sub.x is converted efficiently, compared to a three-way catalyst. Furthermore, lean exhaust aftertreatment devices require that a quantity of reducing material, eg., fuel or ammonia, be present in proportion to the amount of NO.sub.x to be reduced. In U.S. Pat. No. 5,628,186 a method is disclosed in which a reductant is supplied to the exhaust stream of an engine based on output from a sensor in the exhaust line. Optionally, an NO.sub.x sensor can be located in the exhaust line to refine the method. U.S. Pat. No. 5,628,186 does not overcome the narrow temperature range difficulty with lean exhaust aftertreatment devices. In the present invention, two exhaust aftertreatment devices are employed: one in the exhaust line closer to the engine and at least one more downstream in the exhaust line. Because the temperature in the exhaust line decreases along its length, the temperatures in the exhaust aftertreatment devices are dissimilar providing an opportunity for selectively using only a device operating in the appropriate temperature range for acceptable NO.sub.x conversion.
U.S. Pat. No. 5,845,487 discloses a method in which two modes of operation are employed depending on the operating condition of the engine. In a first mode, injection timing is retarded which results in low feedgas NO.sub.x concentration and a penalty in fuel economy. In the second mode, injection timing is advanced which results in a fuel economy improvement at the cost of higher feedgas NO.sub.x. Exhaust stream additive is injected in the latter mode to effect reduction of NO.sub.x in an exhaust aftertreatment device. Retarding injection timing hurts fuel economy. An advantage of the present invention is that it increases the effective window of exhaust gas temperature over which acceptable NO.sub.x conversion efficiency occurs thereby eliminating or limiting the need for fuel inefficient measures as retarding injection timing.
Other strategies have been devised in which engine operating condition is altered in pursuit of exhaust gas temperatures suitable for high NO.sub.x conversion efficiency. It is obvious to one skilled in the art that such measures lead to degradation in fuel economy, driveability, emission control of other pollutants, or a combination thereof. Even if these disadvantages could be overcome, the engine control unit must manage the engine such that a switch between desired operating condition and an engine control point for NO.sub.x reduction is imperceptible to the vehicle operator. The method and apparatus of the present invention overcome these disadvantages by improving the effective temperature window of acceptable NO.sub.x conversion efficiency.
U.S. Pat. No. 5,365,734 discloses a system which employs a plurality of lean NOx catalysts and exhaust conduits. The system includes valve(s) to divert the exhaust flow, with the desired outcome of controlling the space velocity in each of the catalysts to provide high conversion efficiency. A system according to the present invention, in contrast, improves upon U.S. Pat. No. 5,365,734 in that diverting valves and multiple exhaust ducts are not required for handling the exhaust stream. It is known in the art that exhaust ducts are harsh environments in which to place moving parts, such as a valve, which must provide robust and long-lived service. Furthermore, the present invention bases the control of active exhaust aftertreatment devices on temperature, instead of space velocity, as is the case of in U.S. Pat. No. 5,365,734.
U.S. Pat. No. 5,369,956 discloses a system employing an ammonia sensor downstream of the exhaust aftertreatment device in which the supply of exhaust stream additive is controlled so as to avoid breakthrough, i.e., exhausting unreacted ammonia. The present invention provides three advantages: an exhaust stream additive sensor is not required; exhaust stream additive is supplied to provide high NO.sub.x conversion efficiency, as opposed to merely preventing breakthrough; and, supply of exhaust stream additive is altered based on operating temperature, again to provide best NO.sub.x conversion efficiency.
A method is disclosed in U.S. Pat. No. 5,233,934 in which two catalysts are employed in the flue gases from a burner using NO.sub.x and ammonia sensors. Ammonia slip and NO.sub.x emissions are regulated by using nitrous oxide sensors to detect a nitrous oxide concentration. Then, the amount of nitrous oxide is optimized while maintaining ammonia slip below a threshold value. The optimization is achieved by varying reductant injection until the minimum value of nitrous oxide as measured by the sensor is obtained. The amount of reductant injected to locate and achieve this minimum is limited by a measurement of ammonia slip from an ammonia sensor. U.S. Pat. No. 5,233,934 requires feedback control optimization on sensor signals in the exhaust line which provide a delayed signal due to the transit time in the exhaust line. In the case of a burner, operating conditions are constant for long periods of time and may be changed slowly mitigating this disadvantage. However, internal combustion engines, particularly those used in transportation, must react quickly to operator demands. The present invention does not use an optimization procedure and uses an engine model instead. Also, the present invention does not depend on exhaust gas sensors; but, if present, they can be used to refine the engine model.